Add UTC support

[shawmanz32na]

Adds support for displaying selected date/times in UTC instead of local time by specifying a mdp-use-utc="true" attribute on the mdp-date-picker or mdp-time-picker elements.

Underneath the hood, we leverage moment.js's moment#utc() methods to use the same date/time object that we've always used, but just to display that moment in the UTC time zone.

I'm happy to add the necessary documentation/demos as necessary, but I'm not sure where to do that. Give me some direction and I can tie a bow on any necessary documentation/demos.

Closes issue #105

[Georgevik]

+1

[ericsvendsen]

Nicely done. I wish this would get merged - for now I just cloned your branch.